Thermistor

ABSTRACT

A thermistor is formed from a mass of fine particle diamonds, cubic boron nitride, silicon carbide, or mixtures thereof, the particles being doped with boron or beryllium to reduce their electrical resistivity. The thermistor is fabricated by positioning contacts in the mass and sintering at a temperature of at least 1,300* C under pressure conditions such that the particles are stable in the crystalline form they had immediately prior to the sintering step.

United States Patent 1 1 3,735,321

Bovenkerk 1 May 22, 1973 [54] THERMISTOR 3,503,902 3/1970 Shimoda ..252/52O X 3,435,399 3/1969 Gielisse et al ....338/22 R [75 1 Invent 3:3 nmenkerk wmhmgton 3,435,398 3/1969 Gielisse et a] ..338/22 R [73] Assignee: General Electric Company, Primary Examiner-C. L. Albritton Southfield, Mich. Attorney-Allard A. Braddock, Harold J. Holt et al.

[22] Filed: June 18, 1971 [57] ABSTRACT [21] Appl. No.: 154,271

A thermistor 1s formed from a mass of fine particle diamonds, cubic boron nitride, silicon carbide, or mix- [52] US. Cl. ..338/22 R, 29/612, 338/22 SD tut-es thereof, the particles being doped i boron or [51] Ill- Cl. ..H01c beryllium to reduce their electrical resistivity. The Fleld of Search 23, 20, thermistor is fabricated y positioning contacts in the 252/512 29/612 mass and sintering at a temperature of at least l,300 C under pressure conditions such that the particles are [56] References Cited stable in the crystalline form they had immediately UNITED STATES PATENTS prior to the sintering step.

2,609,470 9/1952 Quinn ..338/22 R 8 Claims, 2 Drawing Figures THERMISTOR BACKGROUND OF THE INVENTION This invention relates to thermistors in which the resistance varies with temperature over a very wide temperature range. Thermistors are articles having a negative temperature coefficient of electrical resistance, i.e., as their temperature increases their electrical resistance declines. In general, thermistor materials are electrically insulating at room temperature but become quite conductive at high temperatures. Carbon is a well known example of a material having such a negative temperature coefficient. Abrasive materials such as diamond, boron nitride abrasives, and silicon carbide also possess such negative temperature coefficients.

It has been known to enhance the conductivity of abrasive materials by adding dopants to such materials. A dopant" is a material which, when added to the extent of a few parts per million, changes energy levels for conduction or influences electronic properties. Wentorf et al. U.S. Pat. No. 3,148,161 discloses the production of diamonds having improved electrical conductivity by synthesizing the diamonds under high pressure, high temperature conditions in the presence of such dopants as boron carbide, boron oxide, boron nitride,

' nickel boride, lithium borohydride, etc. Gielisse et al.

US. Pat. No. 3,435,399 discloses the use of a single crystal of doped synthetic diamond in a thermistor device.

Gielisse et al. U.S. Pat. No. 3,435,398 discloses the doping of cubic boron nitride with beryllium, sulfur, selenium, boron, silicon, germanium, etc., and the use of a single crystal thereof in a thermistor device. Both boron nitride abrasive, hereinafter referred to as CBN, and diamond must be formed under conditions of very high temperature and pressure. Silicon carbide, on the other hand, can be formed at atmospheric pressure under high temperature conditions. Thus, in order to add dopant to silicon carbide it is only necessary to have the dopant material, which may be a boron compound, present in the melt.

SUMMARY OF THE INVENTION The present invention is directed to thermistors with integral leads which are responsive to a very wide temperature range. These thermistors use as their material having a negative temperature coefficient of electrical conductivity doped diamond, CBN, and silicon carbide, and mixtures thereof, in the form of powders which are provided with integral electrical leads and sintered at a temperature of at least l,300 C under pressure conditions in which the powder particles are stable in the crystalline form they possessed at the start of the sintering step.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a typical reaction vessel used in making the thermistors of this invention. Three thermistor configurations are illustrated. The thermistors are indirectly heated in this type of reaction vessel.

FIG. 2 illustrates the change of resistivity of a single crystal diamond thermistor as compared with a diamond compact thermistor made in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The principal component of the thermistors of this invention consists of powdered diamond, CBN or silicon carbide, and mixtures thereof. These powders incorporate small proportions of activators or dopants which provide them with a degree of electrical conductivity. As used herein the term doped refers to these activator materials. Boron is the most frequently used dopant but other materials are also active. Dopants are disclosed in U.S. Pat. Nos. Wentorf et al. 3,148,161, Wentorf 3,078,232, Gielisse et a1. 3,435,398, and Gielisse et al. 3,435,399, which are incorporated herein by reference.

It is important that the doped powder have a very small particle size. If the process by which the abrasive is produced does not result in sufiiciently small particles the abrasive is mechanically crushed to about -325 particle size. The invention is practiced in a preferred form by using particles which are not more than 40 microns in diameter. However, particles as large as about microns in diameter can be used. The thermistors of this invention are manufactured by compacting a shaped charge of doped abrasive powder which includes a pair of spaced electrodes or leads for subsequent connection in an electrical circuit. The type of cell preferably used for the compaction step is illustrated in FIG. 1. This cell is indirectly heated by passage of an electric current through the cell parts rather than through the charge in accordance with FIG. 3 of Wentorf et a1. U.S. Pat. No. 3,148,161 or FIGS. 4 and 5 of Bovenkerk U.S. Pat. No. 3,031,269, both of which are incorporated herein by reference. The cell of FIG. 1 is incorporated in a press apparatus of the type disclosed and claimed in Hall U.S. Pat. No. 2,941,248 which is incorporated herein by reference. The compaction or aggregation step is carried out at a temperature of at least l,300 C and under pressure conditions such that the doped material is stable in its very hard abrasive crystalline form. In the case of diamond this pressure is normally in excess of 50,000 kilobars. The pressures and temperatures for diamond are correlated so that sintering takes place in the so-called diamondstable region" as described and claimed in Hall et al. U.S. Pat. No. 2,947,610 which is incorporated herein by reference.

As shown in FIG. 1 a cell 10 consists of a cylindrical shell 11 formed of a material such as pyrophyllite. On the interior of the cylinder 11 is a layer of graphite 12 through which an electrical current is passed during the sintering cycle to provide indirect heating of the contents of the cell 10. On the interior of the graphite cylinder 12 is a cylindrically shaped member 13 formed of alumina, the interior wall of which defines the charge chamber. The ends of the charge chamber are defined by alumina end discs 15 and 18. Overlying these end discs are a pairof alumina end plugs 14 and 14'. Overlying these plugs 14 and 14' are a pair of metal end discs 21 and 21 which are in contact with the ends of the graphite cylinder 12 and, therefore, provide surfaces towhich electrical leads may be connected for the purpose of heating the material undergoing sintermg.

In the illustrated embodiment the interior of the charge chamber includes a pair of alumina partitions 16 and 17 which serve to minimize movement of the charge during the sintering step. The charge chamber of FIG. 1 is shown prior to the application of pressure. It contains a first thermistor 23 with its associated electrical leads 23a and 23b embedded in the abrasive material itself. A second thermistor 24 includes a pair of 5 leads 24a and 24b which are spaced from each other as required by the characteristics desired in the finished product. A third thermistor 25 is illustrated with leads 25a and 25b. The thermistors 23, 24, and 25 have different shapes by way of illustrating that this invention is not directed to any particular shape or configuration. The space in the charge chamber not occupied by the thermistors 23, 24 and 25 is filled with graphite powder or shaped graphite pieces 22 or other finely divided material which will apply the imposed pressure to the thermistor components 23, 24 and 25 and provides a reducing or inert environment for the sintering operation.

The compacting and sintering step is carried out as described in Wentorf et al. U.S. Pat. No. 3,233,988 which is incorporated herein by reference. It is important that the doped abrasive material be subjected to at least one cleaning step before being formed into the shape desired for the thermistor. The electrical leads which are implanted in the doped material must remain solid during the sintering step. Metals such as molybdenum, platinum and tungsten are satisfactory for use as electrical leads. Molybdenum is a preferred material for the leads. However, other metals having good conductivity and high melting temperatures are satisfactory for this purpose and alloys may be used also. The compaction step is carried out under pressure and temperature conditions sufficiently high to compress the powdered material to at least 90 percent of its theoretical density and preferably above 95 percent of its theoretical density.

The polycrystalline thermistors of this invention have characteristics which differ from those of Gielisse et al. U.S. Pat. Nos. 3,435,398 and 3,435,399. FIG. 2 illustrates the different electrical characteristics of single crystal thermistor diamond of the type described and claimed in Gielisse et al. U.S. Pat. No. 3,435,399 and the polycrystalline diamond compact of the present invention. Advantages accruing to the present invention include the ability to provide thermistors having a relatively large quantity of doped abrasive thereby increasing the current rating range together with the fact that leads are buried in the doped abrasive material thereby eliminating the problem of contact resistance and lead strength. The surface area of the lead in contact with the doped material can be readily controlled.

While the invention has been described with particular reference to diamond and CBN abrasive, which must be compacted under extreme pressure conditions, the invention is also directed to silicon carbide which does not require that sintering take place under these extreme pressures. Where mixtures of silicon carbide and the other doped abrasives disclosed herein are used the sintering step must be carried out under high pressure conditions. In this event, care must be taken to avoid the total melting of the silicon carbide.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A thermistor providing linear resistancetemperature characteristics over a range of at least 0 to 800 C comprising a mass of sintered polycrystalline compact selected from the group consisting of doped diamond powder, doped cubic boron nitride powder, doped silicon carbide powder, and mixtures thereof, and a pair of spaced electrodes extending from the interior of said compact, said electrodes serving as leads for connecting said thermistor in an electrical circuit.

2. A thermistor as claimed in claim 1 wherein the powder is doped diamond.

3. A thermistor as claimed in claim 1 wherein the powder is doped cubic boron nitride.

4. A thermistor as claimed in claim 1 wherein the electrodes are composed of molybdenum.

5. The method of making a thermistor which comprises charging a reaction chamber of an ultra-high pressure apparatus with a mass of particles selected from the group consisting of doped diamond powder, doped cubic boron nitride powder, doped silicon carbide powder, and mixtures thereof, positioning a pair of electrodes extending from the interior of said mass in spaced relationship, and sintering said mass at a temperature of at least l,300 C under pressure conditions in the region of the phase diagram in which said particles are stable in the crystalline form they had immediately prior to sintering.

6. A thermistor as claimed in claim 5 wherein the powder is doped diamond.

7. A thermistor as claimed in claim 5 wherein the powder is doped cubic boron nitride.

8. A thermistor as claimed in claim 5 wherein the electrodes are composed of molybdenum.

* III 

2. A thermistor as claimed in claim 1 wherein the powder is doped diamond.
 3. A thermistor as claimed in claim 1 wherein the powder is doped cubic boron nitride.
 4. A thermistor as claimed in claim 1 wherein the electrodes are composed of molybdenum.
 5. The method of making a thermistor which comprises charging a reaction chamber of an ultra-high pressure apparatus with a mass of particles selected froM the group consisting of doped diamond powder, doped cubic boron nitride powder, doped silicon carbide powder, and mixtures thereof, positioning a pair of electrodes extending from the interior of said mass in spaced relationship, and sintering said mass at a temperature of at least 1,300* C under pressure conditions in the region of the phase diagram in which said particles are stable in the crystalline form they had immediately prior to sintering.
 6. A thermistor as claimed in claim 5 wherein the powder is doped diamond.
 7. A thermistor as claimed in claim 5 wherein the powder is doped cubic boron nitride.
 8. A thermistor as claimed in claim 5 wherein the electrodes are composed of molybdenum. 